8 1/4 to 8 3/4 rear diff swap

Ok guys here is my question.I am running a 904 trans with a 7 1/4 rear in my 70 duster.If I swap it out with a 8 3/4 do I have to change the drive shaft or is length and issue.Also thinking of going with 8 1/4 because it should be the same i think.Would greatly appreciate any info.Thanks,Eddie Aa rearend as well.